The present invention is directed to a soft, absorbent and strong paper product, and more particularly, to a high bulk, low dry modulus and high wet strength ratio paper product.
In the manufacture of a number of paper products, such as tissues, towels, napkins, wipers and the like, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product""s intended purposes. Among these various attributes, improving softness, strength, absorbency, bulk and stretch have always been major objectives, particularly for products in the consumer markets. Generally, disposable paper products rely on superior performance in softness, absorbency and strength. In particular, the consumer desires a paper product that is moldable as a cleaning instrument, absorbs large spills and does not tear when wet. In addition, the manufacturer desires a firm paper product that has a low roll weight and a large diameter.
Softness is generally how the paper product feels to the user on his or her face or hand. Softness generally depends on various physical properties, including the surface feel and stiffness of the product. The stiffness, in turn, generally depends on the strength of the product. The strength of the paper product is the product""s ability to maintain its physical integrity and to resist tearing or shredding under use conditions, particularly when wet. Strength is a combination of tensile strength and stretch. When one is higher, the other can be lower and still maintain xe2x80x9cstrength.xe2x80x9d Also, when a certain level of wet strength is needed, using a binder that provides a higher ratio of wet/dry strength allows dry strength to be lower and, therefore, softness to be higher.
Traditionally, many paper products have been made using a wet-pressing process in which a significant amount of water is removed from a wet-laid web by pressing or squeezing water from the web prior to final drying. In particular, while supported by an absorbent papermaking felt, the web is squeezed between the felt and the surface of a rotating heated cylinder, such as a Yankee dryer, using a pressure roll as the web is transferred to the surface of the Yankee dryer. The dried web is then dislodged from the Yankee dryer with a doctor blade, which is known as creping. Creping serves to partially debond the dried web by breaking many of the bonds previously formed during the web-pressing stages of the process. The web may be creped dry or wet. Creping can greatly improve the feel of the web, but at the expense of a significant loss in strength.
A creping method to make both a strong and soft towel is disclosed in U.S. Pat. No. 3,879,257, issued to Gentile et al. and assigned to the Scott Paper Company (1975), entitled xe2x80x9cAbsorbent Unitary Laminate-Like Fibrous Webs and Method for Producing Them,xe2x80x9d herein incorporated by reference. The Gentile et al. patent discloses a process of creping a base sheet, then printing a binder on one side of the base sheet, creping the base sheet again, then printing a binder on the other side of the base sheet, and then creping the base sheet a third time. In particular, the base sheet is printed while traveling through gravure nip rolls. During the gravure print process referred to as the Double ReCrepe (DRC) process, the gravure print process compresses the base sheet to less than 50% of its incoming caliper as it prints the binder onto the sheet. The DRC process provides a web possessing a good combination of strength and softness, but the process of having, successively, three pressings does not provide a particularly bulky sheet. Also, a process that includes three crepes is much more complicated than a process of having one crepe.
More recently, through-drying has become an alternate means of drying paper webs. Through-drying provides a relatively noncompressive method of removing water from the web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from a forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until fairly dry. The resulting through-dried web is bulkier than a conventionally dried creped sheet because the web is less compressed. Squeezing water from the wet web is eliminated, although the use of a pressure roll to subsequently transfer the web to a Yankee dryer for creping may still be used.
While there is a processing incentive to eliminate the Yankee dryer and make an uncreped throughdried product, uncreped throughdried sheets are typically stiff and rough to the touch, if not calendared or layered, compared to their creped counterparts. This is partially due to the inherently high stiffness and strength of an uncreped sheet, but can also in part be due to the coarseness of the throughdrying fabric onto which the wet web is conformed and dried.
Accordingly, there is a need for a paper towel product, or paper sheet, that is soft, absorbent and strong, and more particularly, which has higher bulk, lower dry specific modulus and higher wet strength ratio values than those products made conventionally using a uncreped through-dried process or a double recreped process.
One aspect of the invention provides a strong, soft, bulky and absorbent disposable paper product, or paper sheet, having a dry, specific modulus less than about 0.0040 kilograms/grams per 3 inches, a bulk of greater than about 10 cm3/g and a wet strength ratio of greater than 0.40. Preferably, the specific modulus of the strong, soft and absorbent disposable paper product, or paper sheet, is less than 0.0038. More preferably, the specific modulus is less than 0.0034. Preferably, the bulk of the product or paper sheet is greater than 11. More preferably, the bulk is greater than 12. Preferably, the wet strength ratio of the product or paper sheet is greater than 0.5. More preferably, the wet strength ratio is greater than 0.6. This product or paper sheet also tends to have more dry and wet stretch than most previous products and paper sheets.
In one embodiment, the paper product is manufactured by first producing an uncreped through-air dried base sheet, then printing binder onto one side of the base sheet, then creping that side of the base sheet, and then printing and creping, successively, the other side of the base sheet.
These and other objects, advantages, and features of the present invention will be better understood upon review of the following detailed description of the preferred embodiments.